In 1992, two major journals (Archives of General Psychiatry, Journal of the American medical Association) dedicated entire issues to aggression and violence, including in children and adolescents, underlining it as a major public health concern. Poorly-tested psychotherapeutic agents are administered to children and adolescents, often on a long-term basis, to reduce aggression. Many pressures, including managed care, will increase the utilization of pharmacotherapy in the outpatient setting to treat serious problems. Lithium is the most promising agent for the pharmacotherapy of aggression in children and adolescents, as evidenced by short-term trials. However, it has not been demonstrated that lithium is an effective treatment for aggression in children and adolescents in the outpatient setting, or on a long-term basis. The proposed study is a 2-phased clinical trial of lithium for the treatment of aggression in conduct disorder. Both phases will be double-blind and placebo-controlled, with randomization and employ a parallel groups design. Subjects will be 75 children and adolescents, aged 9 to 17 years, diagnosed with conduct disorder and displaying a profile of severe aggression. Phase 1 will contain a short-term 8-week controlled trial, with twice as many subjects randomized to lithium as placebo, increasing the pool of potential lithium responders to continue to Phase 2. In Phase 2, lithium responders from phase 1 will enter a 6-month long-term controlled trial. Every attempt will be made to define responders to lithium.